1. Field of the Invention
The present disclosure is related to the field of communications and, specifically, to the field of data communication by Frequency Shift Keying (FSK) modulation.
Gaussian Frequency Shift Keying (GFSK) is a bandwidth-efficient type of FSK digital modulation. Specifically, GFSK modulation utilizes a pulse shaping Gaussian filter to reduce the bandwidth of a modulated transmission carrier. In FSK modulation, a data symbol sequence having sharp transitions results in a modulated transmission carrier having discontinuities in frequency. The frequency discontinuities result in a transmission carrier of wide bandwidth. Smoothing the sharp transitions of the data symbol sequence, however, using a pulse shaping Gaussian filter, circumvents this problem. The pulse shaping Gaussian filter removes the higher frequency components in the data symbol sequence which, in turn, permits a more compact transmission spectrum.
The compact transmission spectrum facilitated by the GFSK modulation scheme aides wireless communication systems which operate in both licensed bands and the unlicensed industrial, scientific, and medical (ISM) bands, by reducing the spectral bandwidth and out-of-band spectrum of the GFSK transmission carrier, to meet FCC adjacent channel power rejection requirements. Similar requirements are enforced by international radio spectrum regulatory bodies.
However, pulse shaping by the pulse shaping Gaussian filter induces inter-symbol interference (ISI). In fact, it is the pulse shaping by the Gaussian filter which introduces the ISI. Consequently, systems designed around the GFSK modulation scheme are designed in view of low data throughput or increased bit error rate. Conventionally, the ISI associated with the GFSK modulation scheme prohibits data communication at high modulation orders, where multiple bits of data are transmitted per symbol. In an attempt to facilitate GFSK communications systems with higher data throughput, the use of more complex and expensive receiver structures have been proposed.
2. Discussion of the Related Background Art
As illustrated at FIG. 1, a first related art GFSK system 100 includes a GFSK transmitter 102 and a GFSK receiver 114.
The GFSK transmitter 102 includes a data source 104, Gaussian filter 106, an FSK modulator 108, a transmitter back end 110, and a transmission antenna 112. The Gaussian filter 106 filters a data symbol sequence provided from data source 104, and outputs a pulse-shaped data symbol sequence to the FSK modulator 108. The FSK modulator 108 modulates a carrier frequency based on the pulse-shaped data symbol sequence, according to a selected FSK modulation order (i.e., a number of bits per symbol). The output of the FSK modulator 108 is provided to the transmitter back end 110, where it is up-converted to a transmission frequency and coupled to the transmission antenna 112 for radio-frequency (RF) transmission. Accordingly, the transmission antenna 112 transmits a GFSK modulated transmission carrier.
The GFSK receiver 114 includes a reception antenna 116, a receiver front end 118, a channel filter 120, a discriminator 122, a post detection filter 124, a symbol slicer 126, and a data sink 128. In operation, the reception antenna 116 and receiver front end 118 receive a transmitted GFSK modulated signal and down-convert the received GFSK modulated signal to baseband. The channel filter 120 selectively filters the received baseband GFSK modulated signal to reject adjacent channel interference and Additive White Gaussian Noise (AWGN). The discriminator 122 performs frequency demodulation by providing an output signal that is proportional to the instantaneous frequency of the modulated transmission carrier and outputs a demodulated sequence of symbols. Specifically, in the case of a 1 bit/symbol modulation order (i.e., 2-GFSK), the discriminator 122 discriminates between two frequencies, f0+f1 and f0−f1, where f0 is the un-modulated carrier frequency. The post detection filter 124 filters the demodulated sequence of symbols produced by the discriminator 122 to reduce noise amplified by the discriminator 122. The slicer 126 produces symbol decisions based on the filtered sequence of symbols output from the post detection filter 124, to produce a sequence of symbol decisions, which is provided to the data sink 128. In the GFSK receiver 114, the post detection filter 124 is not designed to remove ISI, and the slicer 126 is required to produce symbol decisions in the presence of ISI, causing symbol and bit errors to occur.
In the first related art GFSK system 100, the ISI introduced by the Gaussian filter 106 requires that a modulation scheme of low modulation order (i.e., few bits/symbol) be used by the FSK modulator 108. Otherwise, unacceptable levels of symbol and bit errors will occur at the GFSK receiver 114. Specifically, the ISI introduced by the Gaussian filter 106 causes the “eye” of the demodulated symbol sequence output by the discriminator 122 to close, and, thus, the slicer 126 will produce erroneous symbol decisions, as the output of the discriminator 122 will fail to be consistently above or below symbol decision threshold(s) of the slicer 126 with certainty at determined symbol timings. At higher modulation orders, it becomes even more difficult for the discriminator 122 and the slicer 126 to produce correct symbol decisions. Therefore, the data throughput of the first related art GFSK system 100 is limited because of the ISI introduced by the Gaussian filter 106, as only lower order modulation schemes may be utilized without unacceptable levels of symbol errors. The channel filter 120 also contributes to the introduction of ISI in the received signal, further compounding the limitations of the GFSK system 100.
As illustrated at FIG. 2, a second related art GFSK system 200 includes a GFSK transmitter 202 and a GFSK receiver 214.
The GFSK transmitter 202 includes a data source 204, a Gaussian filter 206, an FSK modulator 208, a transmitter back end 110, and a transmission antenna 212. The GFSK transmitter 202 operates the same as the first related art GFSK transmitter 102.
The GFSK receiver 214 includes a reception antenna 216, a receiver front end 218, a channel filter 220, a discriminator 222, a maximum likelihood sequence estimator (MLSE) 224, and a data sink 226. As compared to the first related art GFSK receiver 114, the second related art GFSK receiver 214 relies upon the MLSE estimator 224 to produce symbol decisions in the presence of ISI. That is, the MLSE estimator 224 does not remove the ISI. Instead, the MLSE estimator 224 estimates data symbols according to a least probability of errors, in the presence of the ISI, and outputs data bits in terms of error probability. For example, the MLSE estimator 224 may utilize the Vitrerbi algorithm for determining a symbol decision of lowest error probability, attempting to mitigate the presence of the ISI. However, especially at low signal-to-noise ratios (SNR), MLSE estimators cannot adequately mitigate symbol errors due to ISI.